1. Field of Invention
The present invention relates to a transmitter, a communication system and a communication method thereof.
2. Description of Prior Art
A technique for transmitting and/or receiving by using multiple antennas or antennas array (MIMO, Multiple Input and Multiple Output) is one of core technologies for the next generation broadband wireless communication system. A MIMO communication system uses Nt transmitting antennas and Nr receiving antennas to transmit data in parallel, and forms a MIMO channel matrix of size Nr×Nt which may be divided into L separately parallel space sub-channels, where L<=min(Nr,Nt), in which min(Nr,Nt) means to select the minor number of number Nt and Nr. The separate data flow can be transmitted in parallel on the L separate space sub-channels in order to improve spectrum efficiency of the system, where Nr and Nt represent the numbers of receiving and transmitting antennas, respectively.
The separately parallel space sub-channels in the MIMO system don't have the same communication conditions. In particular, a channel gain, interference and fading of each space sub-channels are different. Moreover, the difference is generally significant. Therefore, a data transmission rate supported by each space sub-channels is different, which is also effected by a transmitting manner, a QoS (Quality of Service) requirement and a transmission power. As the communication conditions change, the system performances obtained by different signal transmission manners are different. For example, in the case of low signal-noise-ratio environment or the communication condition with line of sight (LOS), a beam-forming transmission can achieve a broader range of performance optimization. In other words, in the case of the channel environment with a high sub-channel gain, a beam-forming transmission is optimal; a correlation among antennas reduces performance of a Spatial-Temporal Coding system. A transmission signal processing performed by using the Channel State Information obtained from the Transmitter (CSIT) can further improve performance of the MIMO system. A transmission pre-coding or multi-mode transmission beam-forming is one of such techniques. Since by using CSIT, the transmitter may obtain the channel condition or fading status of each space sub-channels, the transmission power allocation, the transmission symbol bit allocation, the constellation selection, etc. can be performed according to knowledge about those space sub-channels, causing the performance of the MIMO system to be improved. However, in the case that it occurs a lager error in the CSIT obtained by the transmitter, the performance of the transmission pre-coding system will be significantly reduced on the low-gain sub-channel. Generally speaking, the system gain to be obtained by the transmission pre-coding technique is highly associated with the accuracy of CSIT obtained by the transmitter. The beam-forming or the single-mode transmission technique has a broader optimization range in the MIMO wireless environment with high-gain sub-channels or with LOS. On the other hand, the STC transmission technique needn't any channel state information.